Retroreflectors reflect incident light back to the light source. Retroreflectors can be used in signs, such as, for example, construction signs and signposts; in noticeable tape, such as, for example, tape for an automobile or motorcycle; in clothing; in safety materials, such as, for example, life-saving devices; in signboard markings; in reflective boards for visible and laser light; and in infrared light reflecting sensors. Retroreflectors including cube corners are often considered far superior in retroreflective efficiency as compared to retroreflectors including glass beads. Consequently, the use of cube corner retroreflectors is increasing annually.
Cube corner retroreflective sheeting generally fall into two categories: (1) sheetings having a retroreflective structure on a front surface on which light is incident and (2) sheetings having a retroreflective structure on a rear surface. Most commercially available cube-corner retroreflective sheetings are of the latter type. Light is incident on the planar front surface, passes through the thickness of the body layer, and is retroreflected by the retroreflective structure of the rear surface to return to the front surface. In some cases, a reflective coating such as aluminum is applied to the rear structured surface, followed by an adhesive layer that, to some extent, covers and conforms to the shape of the structured surface. However, a reflective coating is not required as long as a clean air interface can be maintained at the structured surface, in which case reflections occur by total internal reflection.
The cube-corner retroreflective sheeting described in Japanese Patent Kohyo Publication 2002-535691 and U.S. Pat. No. 6,280,822 includes a retroreflective structure on its front surface. By laminating a thin metal film on the structured front surface, it is possible to enhance the reflectance of the surface. Incident light is reflected on plural surfaces forming the retroreflective structure, and does not penetrate the body layer. The retroreflective structure is formed by the arrangement of multiple cube corner structures, such as, for example, cube-corner type trigonal pyramids, cube-corner type cavities, or both. “Cube-corner type trigonal pyramid” means a projecting portion of a trihedral prism form having three exposed planes. “Cube-corner type cavity” means a recessed portion of a trihedral prism form similar to that mentioned above.
Japanese Patent Kokai Publication H5 (1993)-346767 describes an infrared light reflecting cube-corner sheeting. In this cube-corner sheeting, the polymer that forms the cube-corners contains a colorant that is transparent for infrared light but is substantially opaque for visible light. Such cube-corner sheeting can retroreflect incident infrared light while absorbing visible light. Thus when the cube-corner sheet is used as an infrared reflective board, any color or image underneath the sheeting cannot be seen. This type of sheeting, therefore, is inadequate for use where design or aesthetics are important.
Japanese Patent Kohyo Publication 2002-509276 and U.S. Pat. No. 6,157,486 describe a wavelength-selective retroreflector. In this retroreflector, a multilayered reflective film having wavelength-selective transmissibility is disposed on the surface of a retroreflective article. The reflector only retroreflects light within a specific wavelength range, as determined by the multilayered reflective film chosen, and specularly reflects all other light incident on the reflector. The selective retroreflection of infrared light and the specular reflection of visible light causes the retroreflector to exhibit a mirror-like appearance. Consequently, colors or images underneath the retroreflector cannot be seen.